In the field of orthopaedics, it is well known to secure various types of implant components with screws. For example, a prosthetic hip cup may include a hemi-spherical shaped outer metal shell which is screwed into the prepared acetabulum. An inner mating articulating surface (often made of a plastic such as ultra high molecular weight polyethylene, UHMWPE) is then secured to this outer shell. A prosthetic femoral hip stem then typically mates with this inner articulating surface of the hip cup. Alternatively, the tibial component of a prosthetic knee joint may include a metal support plate which is screwed on to the prepared proximal tibia. A mating tibial articulating surface, which again may be UHMWPE, is then secured to the support plate. A prosthetic femoral knee component then typically mates with this tibial articulating surface.
It has been discovered that over time particulate matter or wear debris may be generated from the articulation of this prosthetic load bearing joint, and that this particulate may pass beneath the UHMWPE articular surface and move down along the screw head and shaft causing bone lysis. It is known to provide polyethylene plugs to block the screw hole openings in base members such as tibial plates if screws are not utilized through such holes to prevent the particulate from passing through the holes. However, if the screws are used, the plugs are removed and then the particulate may then pass through the hole and move down along the screw.
In other implants, screws may be utilized to either secure components to bone, such as with bone plates, or to secure directly to the implant component, such as in U.S. Pat. 4,875,474 to Border in which an end cap is threadedly secured to an intramedullary nail. Border discloses a seal or washer 264 shown in FIG. 12 and described at column 7, lines 19-24 and 50-56. The washer 264 is provided to form a seal between the bottom surface of head 266 of end cap 260 and the end of proximal portion 212 of the nail. This seal is indicated to prevent the ingrowth of tissue (into the opening in the nail) during the implantation period. The seal of Border is beneath the flat surface of the head of the end cap, and is limited to the axis of the shaft of the cap being perpendicular to the top flat surface of the substrate (the nail or base portion) for the washer to seal properly. In other words, the axis of the shaft of the cap must be aligned or coincident with the central axis of the hole in the nail.